Kostya's Boring Codec World

A recent question about buggy SMC decoding led me deep into QuickTime specification to look at the codec missing opcode. And there are some noteworthy things here as well.

Back in the day there was the multimedia player for Unix called XAnim. Its last release was in 1999—before other opensource multimedia player projects have started! It was both feature-rich (e.g. it could step frames forward and backwards, something that not all current media players can do) and had an excellent codec support for the time.

Somehow its author reverse engineered (long before the era of decompilers too) a lot of codecs and somehow managed to obtain the sources for e.g. Indeo and while he could not provide them, he offered them for a wide variety of architectures—Alpha, MIPS, Sparc, PowerPC, x86. It was a treasure trove for formats and lots of the decoders were ported to other projects (even I did that for one or two codecs) and binary codecs were a great help in reverse-engineering efforts as well.

Now to SMC itself. Formally it’s QuickTime Animation codec but people call it after its FOURCC which is “smc “, probably after the author’s initials.

Opensource SMC decoders come from the same source (I based mine on the description in The Wiki but you can guess what that description is based on; and yes, back in the day e.g. MPlayer and Xine had their own decoders for various codecs before relying on libavcodec for everything). After looking at the binary specification I can say it looks exactly like it was reverse engineered from it directly (it has the same logic and data types but lacks sensible names). Anyway, the thing is that it does not handle opcode 0xF0 and I finally had an occasion to look at it.

I took QuickTime 6.3 binary specification for Windows (somehow the decoder ended in QuickTimeInternetExtras.qtx) and looked inside. It turns out that there are several decoding functions there (for different output formats) but they all do the same: handle 0xF0 opcode in exactly the same way as 0xE0 opcode (raw blocks), there are no differences there whatsoever.

That’s one mystery less, even if the answer is a bit disappointing. At least I could reminisce about good old times hardly anybody else remembers.

This entry was posted on Tuesday, August 10th, 2021 at 1:10 pm and is filed under Various Video Codecs. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.